In modern wireless systems, such as for example cellular wireless and fixed radio access wireless networks, there is a need for equipment operating with radio frequency signals, such as radio transceiver equipment in user equipment or at base stations or access points, which is economical to produce, while having high performance at radio frequencies. Increasingly high radio frequencies are being used as spectrum becomes scarce and demand for bandwidth increases. Furthermore, antenna systems are becoming increasingly sophisticated, often employing arrays of antenna elements to provide controlled beam shapes and/or MIMO (multiple input multiple output) transmission. Typically, radio frequency circuits are constructed with unbalanced transmission lines for transmission of radio frequency signals between components of the circuit, such as between amplifiers, filters, detectors, antennas and many other types of radio frequency component. An unbalanced transmission line comprises a conductor, such as a printed track of a circuit board, and a ground structure, such as a ground plane of a circuit board or a conductive ground plate, such as a milled aluminium plate. If one transmission line is connected to another transmission line, it is important that there is a good radio frequency connection between the ground structures of the respective transmission lines. This ensures a low loss connection between the transmission lines and, by ensuring that the conductors are referenced to the same ground voltage, reduces pick up of spurious signals. Conventionally, ground structures may be connected together at radio frequency using the outer conductors of co-axial radio frequency connectors, opposite mating parts of which are connected to the respective ground structures. Such co-axial connectors are typically made to tight mechanical tolerances and are relatively expensive, often being gold plated for example to ensure a good electrical connection and avoid corrosion. Alternatively, or in addition, ground structures may be connected at radio frequency by soldering together or by mechanical fixing of one structure to the other, for example by screws. However, such connections may be expensive to assemble and may require production of mechanical components to tight tolerances. Furthermore, a poorly assembled or corroded direct mechanical attachment between ground structures may result in a poor radio frequency connection resulting in loss or even a connection with non-linear transmission characteristics resulting in generation of spurious signal components. In particular, if a ground plane, for example a conductive layer of a multi-layer circuit board, is to be attached to a ground plate, such as a backing plate of an antenna or antenna array, there is typically a need to connect one or more signals that are referenced to the ground plate to components on the circuit board which are referenced to the ground plane of the circuit board. In this case there is a need for a low cost, high performance radio frequency connection, preferably tolerant of mechanical misalignment, between a transmission line referenced to the ground plate and a transmission line referenced to the ground plane.
It is an object of the invention to mitigate the problems of the prior art.